Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Gas vs Vapour

  1. Jun 14, 2009 #1

    danago

    User Avatar
    Gold Member

    What is the difference between a gas and a vapour? I remember one of my lecturers saying that there was a slight technical difference, but i dont think he ever did explain it because it was not important for what we were doing.

    I did a quick google search and one of the results i found was that 'vapour' refers to a substance in the gaseous phase even though under normal conditions it does not exist as a gas i.e. the vapour pressure of a liquid/solid at standard conditions, whereas a 'gas' refers to a gaseous substance that does naturally occur as a gas under standard conditions, such as oxygen or nitrogen.

    Is this distinction correct? The source wasn't the most reliable of sources, so it would be nice to get some validation :smile:

    Thanks,
    Dan.
     
  2. jcsd
  3. Jun 14, 2009 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    A gas is one of the states of matter: solid, liquid, and gas. Anything can be any one those depending on the temperature but things like oxygen, carbon dioxide, etc. are gases at "standard" temperature and pressure. "Vapor", however, is liquid droplets suspended in air.
     
  4. Jun 14, 2009 #3

    drizzle

    User Avatar
    Gold Member

    sorry off topic, but isn't the plasma considered as a fourth state of matter?
     
  5. Jun 14, 2009 #4

    MATLABdude

    User Avatar
    Science Advisor

    And a Bose-Einstein condensate is considered a fifth, thus allowing you to span the entire temperature spectrum.
     
  6. Jun 14, 2009 #5

    drizzle

    User Avatar
    Gold Member


    good, where can I read a good description of Bose-Einstein condensate? I would like to know, thanks in advance
     
  7. Jun 15, 2009 #6

    Borek

    User Avatar

    Staff: Mentor

    Yes and no. When we speak about vapor pressure (or saturated vapor pressure) we think about gas.
     
  8. Jun 15, 2009 #7

    MATLABdude

    User Avatar
    Science Advisor

  9. Jun 15, 2009 #8

    danago

    User Avatar
    Gold Member

    Oh, so when we talk about a vapour, it technically isnt in the gaseous phase at all then? Its technically still a liquid, just in a form that appears as a gas?
     
    Last edited: Jun 15, 2009
  10. Jun 15, 2009 #9

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    That's not right. The liquid droplets suspended in the air are condensed water vapor. Condensed water vapor = clouds, uncondensed water vapor = humid air. Big difference.

    A vapor is a substance that is in the gaseous phase but whose temperature is below the substance's critical point.
     
  11. Jun 15, 2009 #10

    danago

    User Avatar
    Gold Member

    So in the case of water, any steam that exists below ~374 degrees celcius would technically be considered a vapour?
     
  12. Jun 15, 2009 #11

    Mapes

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Whoa! As D H, says, vapor is a gaseous phase.

    (So why would we have two words for the same thing? People often use "vapor" to describe a gas in equilibrium with the corresponding liquid or solid phase. But others call this "saturated vapor" to be precise, and say "vapor" to imply only that the liquid or solid phase is present.)
     
  13. Jun 15, 2009 #12

    Ygggdrasil

    User Avatar
    Science Advisor

    You could also look at varying the pressure and include things like supercritical fluids as a state of mater as well.
     
  14. Jun 16, 2009 #13
    When we asked this question in the class, prof gave us a reasonable answer. Gas is something which obeys the ideal gas laws, whereas vapor actually dont, while both are in gaseous phase.
     
  15. Jun 16, 2009 #14

    Borek

    User Avatar

    Staff: Mentor

    As long as there are no traces of condensed liquid, vapor obeys ideal gas law as good as any gas does - classic example will be calculation of amount of water vapor in the equilibrium with liquid. Once the droplets appear, ideal gas law is no longer applicable to the whole system (although it still describes gaseous phase).
     
  16. Jun 16, 2009 #15

    Mapes

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Unfortunately, this is something either your professor made up, or it's a convention in extremely limited use. Vapor in many cases is modeled perfectly accurately with the ideal gas law. An example is the two huge fields of physical vapor deposition and chemical vapor deposition in microfabrication, where the vapor is typically at relatively low pressure and behaves ideally.
     
  17. Jun 16, 2009 #16
    engineering approximations:wink:
     
  18. Jun 18, 2009 #17

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It depends upon how you define 'vapor'.

    eg.
    http://www.thefreedictionary.com/vapor

    It seems that 'vapor' is an imprecise term.
     
  19. Jun 18, 2009 #18
    If you had 1L of liquid water at 25C and 0 Pa in 2L tank with nothing else, would water vapor be present?
     
    Last edited: Jun 18, 2009
  20. Jun 18, 2009 #19

    Mapes

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    A few more vapor definitions: http://books.google.com/books?id=GC...vapor&lr=&as_brr=1&ei=33g6Sq_QKKGeygSGir26BQ". The scientific definition would seem to match the second part of the popular definition (a situation analogous to the scientific and popular definitions of the word theory).

    Yes. The second law tells us that there's a tremendous driving force for water molecules to evaporate, as this increases total entropy (up to the saturation pressure).
     
    Last edited by a moderator: Apr 24, 2017
  21. Jun 18, 2009 #20
    I'm not sure I agree with this. Just because there is a mechanism for an increase in entropy doesn't necessarily mean it will cause a spontaneous reaction. Wouldn't the water molecules first require some sort of catalyst, such as dust or some sort of particle, to actually separate from the liquid and become a vapor. For example, the same way you can super heat water. If there is nothing to help trigger the phase change of water going from liquid to gas then the water will not boil. Wont this same phenomenon be observed with a saturated liquid at STP?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Gas vs Vapour
  1. Be no gas? (Replies: 10)

  2. Vapour pressure (Replies: 9)

Loading...